Time delay ferrule-type fuses commonly have heretofore comprised a cylindrical housing of insulating material having ferrule or cup-shaped terminal extending over the initially open outer ends of the cylindrical housing. Connected between these terminals within the housing are a series of longitudinally spaced elements including current-heatable, short circuit protection means, a thermal mass in heat communication with the current-heatable means, and a current overload heat-meltable connection between the thermal mass and an adjacent electrical portion of the fuse placed under spring tension. When the thermal mass accumulates sufficient heat under a modest overload (e.g. 135% overload) to melt the heat meltable connection, the spring force pulls the connection apart quickly to separate current-carrying portions of the fuse. In prior art fuses, the housing was divided commonly into a pair of outermost, sand or powder-filled, short circuit protection element-containing compartments and a central, time delay overload protection element-containing compartment. These compartments were generally defined by fiber washers spaced from the end terminals and press-filled into the housing. The central compartment generally contained the thermal mass and a conductive element connected through an overload current heat meltable connection.
These three-compartment fuses had a number of disadvantages. First of all, it was mistakenly believed that adequate dielectric strength for short circuit protection required a pair of powder or sand-filled, short circuit element containing compartments. For this reason and because of the design of the elements of the central time delay overload protection element-containing compartment these fuses comprised an undue number of elements, many of which required hand assembly operations. Also, the positions of the fiber washers sometimes shifted causing clearance spaces through which the sand or powder in the outermost compartments leaked into the central compartment.
As will appear, the present invention provides a fuse where the number of parts and solder connections are materially reduced from that required in that the 3-compartment fuse. More importently, the elements within the fuse are designed and related so that they can be assembled by entirely automated equipment. Also, the unique design of the invention forms a more reliable fuse.